Your toenails may hold clues to your risk of developing lung cancer, a new study finds.

The results show men with high levels of nicotine in their toenails were about 3.5 times more likely to develop lung cancer than those with lower levels of nicotine, regardless of their smoking histories.

The findings suggest the detrimental effects of smoking may be underestimated in studies that use only self-reported smoking history to assess lung cancer risk, the researchers said.

Studies that only ask people how much they smoke might misjudge the amount of nicotine people actually inhale — either because people fudge a little when they answer, or because some may smoke fewer cigarettes than others but inhale more deeply, taking in more tobacco carcinogens.

To find an indicator of tobacco exposure that would be less prone to error, study researcher Wael Al-Delaimy of the University of California, San Diego, and colleagues turned to toenails. Because toenails grow slowly, nicotine levels in toenail clippings are relatively stable compared to levels in our salvia and urine.

The study involved men who had answered a health questionnaire in 1986, and were followed up every two years. In 1987, 33,737 men provided toenail clippings.

Of these, 210 men developed lung cancer between 1988 and 2000. The researchers compared the toenail clippings of this group to those 630 men who did not develop lung cancer.

The clippings were used to assess the men's tobacco exposure over the last year.

Not surprisingly, participants' smoking history predicted their lung cancer risk.

The nicotine levels in toenail clippings predicted the men's risk of developing lung cancer, regardless of how much participants said they had smoked in the past, and whether they were currently smoking. In fact, the researchers found more than 10 percent of men who had the highest levels of nicotine in their toenail clippings had never smoked.

The findings suggest that "previous studies that have determined risk of lung cancer from tobacco use by using only reported active smoking may have underestimated the true effects of tobacco smoke," the study said.

The researchers noted that nicotine is not a carcinogen, but the findings are based on the assumption that higher levels of nicotine correspond to higher levels of exposure to carcinogens in tobacco.

Nicotine levels could serve as a possible biomarker for exposure to tobacco, and letting someone know their levels might motivate them to quit smoking, or avoid heavy exposure to secondhand smoke, the researchers said.

The work was published online March 2 in the American Journal of Epidemiology.

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